The present invention relates to a motor controller and an electric power steering apparatus.
In many cases, conventional motor controllers used, for example, in electric power steering apparatuses (EPS) include anomaly detecting means. The anomaly detecting means detects an anomaly when flow of electric current fails in any one of U, V, and W phases, due to a break in power supply lines or damage to contacts of a driver circuit. When such an anomaly is detected, control of operation of a motor is quickly stopped and thus fail safe is performed.
However, in the EPS, if the control of the operation of the motor is suspended, the steering characteristics are greatly changed. Specifically, increased steering force becomes necessary for the driver to accurately manipulate a steering wheel. In this regard, Japanese Laid-Open Patent Publication No. 2003-26020, for example, discloses a motor controller that, even if a failure of electric current flow is detected in a certain phase, continuously controls the operation of a motor by employing two phases free of failure of electric current flow as electric current flowing phases. In this manner, assist force is continuously applied to a steering system, thus preventing increase of load on the driver caused by the failsafe.
However, with reference to FIG. 15, if a sinusoidal electric current is supplied to each one of the electric current flowing phases in the above-described conventional case, a torque ripple is caused and thus steering comfort decreases. FIG. 15 illustrates an example in which the electric current flow has failed in the U phase while normal electric current flows are ensured in the V and W phases.
Specifically, as illustrated in FIG. 16, if change of a motor electric current in conventional two-phase drive operation is represented in a d/q coordinate system, a q-axis current command value, which is a target control value of motor torque, remains constant. However, an actual q-axis current value changes in accordance with a sinusoidal wave. In other words, the generated motor electric current does not correspond to requested torque. The motor is thus continuously operated without achieving its full output performance.
Further, as disclosed in Japanese Laid-Open Patent Publication No. 2006-67731, anomaly of a control system, such as generation of an overcurrent caused by a failed driver circuit or a problem of a sensor, is detected through comparison between an electric current deviation (a q-axis current deviation, mainly) of a d/q coordinate system and a predetermined threshold value. However, in the above-described two-phase drive operation, the electric current values of the d-q coordinate system each change in accordance with a sinusoidal wave. The electric current deviation is thus caused regardless of whether there is an anomaly. As a result, such anomaly cannot be detected in the two-phase drive operation.